1. Field of the Invention
The present invention relates to a method of driving a galvanometer mirror, and more particularly to a method of driving a galvanometer mirror in reciprocating strokes at variable periods.
2. Description of the Prior Art
Galvanometer mirrors have widely been used as light deflectors in various optical scanning devices. As is well known in the art, the galvanometer mirror is angularly moved in reciprocating strokes when a sawtooth current is supplied to drive the galvanometer mirror. When the galvanometer mirror is to be reciprocated at a constant period, the drive current used should be of a sawtooth waveform rising and falling in regular periodic cycles. For two-dimensional scanning of a surface, a light beam is deflected by the galvanometer mirror for scanning the surface in one direction. This scanning mode is referred to as "primary scanning". The surface itself is moved in a direction substantially normal to the main scanning direction. This scanning mode is referred to as "secondary scanning". When it is desired in such two-dimensional scanning to vary the rate or speed of the secondary scanning on a given surface being scanned, it is required to change the period of reciprocating angular movement of the galvanometer mirror.
One conventional way for achieving such a variable period of reciprocating movement of the galvanometer mirror is illustrated in FIG. 3 of the accompanying drawings. A voltage having a sawtooth waveform So is applied for angularly moving a galvanometer mirror in reciprocating strokes. The sawtooth voltage has cyclic peaks or maximum values each limited to a voltage V.sub.1 at which the galvanometer mirror is swung or angularly displaced through a maximum angle .theta..sub.1 at one end of its reciprocating stroke. The time intervals T.sub.1, T.sub.2 in which the voltage is kept at V.sub.1 are varied to change the times for which the galvanometer mirror remains at the maximum angular displacement, thereby varying the period of reciprocating angular movement of the galvanometer mirror. FIG. 4 illustrates another method in which the minimum values of the sawtooth waveform So are limited to a voltage V.sub.2 at which the galvanometer mirror is angularly displaced through a maximum angle .theta..sub.2 at the other end of the reciprocating stroke.
The angular displacement of the galvanometer mirror usually has a response delay, due primarily to inertia and friction, with respect to the applied drive voltage as indicated by the dotted lines in FIGS. 3 and 4. Therefore, when the time in which the voltage is kept at V.sub.1 or V.sub.2 is selected to be the shorter time interval T.sub.1 as shown in FIGS. 3 and 4, the galvanometer mirror is caused to swing back before it reaches the maximum displacement .theta..sub.1 or .theta..sub.2, and never arrives at one stroke end. As a result, the stroke of reciprocating movement of the galvanometer mirror is varied to change the times t.sub.1, t.sub.2 for which the light beam deflected by the galvanometer mirror is scanned along straight lines, thus making the lengths of the scanning lines irregular.